1. Field of the Invention
The present invention relates to a semiconductor element that can be used as a switching element or an amplifier element (for example, an organic transistor). In addition, the present invention relates to a light-emitting device with the use thereof.
2. Description of the Related Art
It has been promoted to develop a light-emitting element where electrons are injected from one electrode and holes are injected from the other electrode into a stacked body containing an organic compound sandwiched between the pair of electrodes to recombine the electrons and holes in the stacked body so that a light-emitting material in the stacked body is excited and thus light emission can be obtained.
Being a self-light-emitting type, the light-emitting element is superior in visibility with low dependence on a viewing angle and a thin shape and weight saving can be realized easily. Therefore, attention is focused on the use of a flat panel display of the next generation. In addition, it is also possible to manufacture an element in a flexible film such as plastic, the usage of which is expected as a mobile display.
A light-emitting device using this light-emitting element can be roughly divided into two types, that is, a passive matrix type and an active matrix type. In the active matrix light-emitting device, a transistor is electrically connected in each pixel to control light emission of a light-emitting element.
Thus far, an inorganic semiconductor material typified by silicon has been used for a transistor of the active matrix light-emitting device. However, it is necessary to process at high temperature in order to form the inorganic semiconductor material typified by silicon as a semiconductor layer; therefore, it is difficult to use a flexible material such as plastic or a film for a substrate.
On the other hand, a transistor in which an organic semiconductor material is used as a semiconductor layer can be formed even at comparatively low temperature; therefore, it is possible to manufacture in principle a transistor not only over a glass substrate but also over a substrate having low heat resistance such as plastic.
In such a manner, as an example of a field effect transistor in which the organic semiconductor material is used as a semiconductor layer (hereinafter, referred to as an “organic transistor”), a transistor in which silicon dioxide (SiO2) is used as a gate insulating layer and pentacene is used as a semiconductor layer (see the following Reference 1: Y. Y. Lin, D. J. Gundlach, S. F. Nelson, T. N. Jackson, IEEE Electron Device Letters, Vol. 18, pp. 606-608 (1997)) can be given. In this report, it is reported that field effect mobility is 1 cm2/Vs and transistor performance comparable to amorphous silicon can be obtained even when the organic semiconductor material is used as a semiconductor layer.
An active matrix light-emitting device in which a light-emitting element is driven using this organic transistor is also proposed. Further, there are several reports regarding an organic transistor in which holes are injected from a source electrode and electrons are injected from a drain electrode into an organic semiconductor layer thereof to recombine the holes and electrons in the semiconductor layer and thus light emission is obtained from the organic semiconductor layer itself (hereinafter, referred to as an organic light-emitting transistor) (see Reference 2: M. Ahles, A. Hepp, R. Schmechel, F. v. Seggern, APPLIED PHYSICS LETTERS, Vol. 84, No. 3, pp. 428-430 (2004) and Reference 3: T. Sakanoue, E. Fujiwara, R. Yamada, H. Tada, Chemistry Letters, Vol. 34, No. 4, pp. 494-495 (2005), for example).
Since these organic light-emitting transistors are elements having both functions of a transistor and a light-emitting element, it is considered that the organic light-emitting transistors are advantageous in an aperture ratio compared with a case of manufacturing a transistor, which drives a light-emitting element, separately from a light-emitting element. In addition, since a manufacturing element is reduced compared with the case of manufacturing both a transistor and a light-emitting element, it is considered that the organic light-emitting transistors are advantageous also in a yield or a manufacturing cost of a product.
In the meantime, in the organic light-emitting transistor, holes and electrons have to be injected into a semiconductor layer from a source electrode and a drain electrode in order to obtain light emission; however, there are such problems that light emission does not occur well, a transistor characteristic such as carrier mobility is decreased, or a drive voltage is increased when there is an energy barrier in the interface.
The energy barrier in injecting holes and electrons into a semiconductor layer depends on a relation between a material used for electrodes and an organic semiconductor material and largely influences a work function of the material used for electrodes. Therefore, an electrode material that allows holes and electrons to be injected into a semiconductor layer efficiently, and that can reduce a drive voltage has an extremely narrow option.
Further, in forming a metal electrode, there is a case where the work function may be changed due to etching in forming the electrode or a case where the lower layer may be deteriorated; thus, it is not easy to obtain an organic light-emitting transistor having a low drive voltage which can be manufactured simply and easily.